powerpc/test_emulate_step: Add negative tests for prefixed addi

testcases for `paddi` instruction to cover the negative case,
if R is equal to 1 and RA is not equal to 0, the instruction
form is invalid.

Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200626095158.1031507-3-bala24@linux.ibm.com